Field of the Invention
The present relates, in general, to magnetostrictive position transducers.
The phenomenon of magnetostriction has been widely employed in linear distance and position measuring devices. A magnet located near or around a magnetostrictive wire marks the location to be measured. Such devices can operate with either mechanical or electrical excitation. When an acoustical/mechanical strain propagating along the wire reaches the area of influence of the magnet, an electrical signal is generated in the wire. Conversely, when an electrical signal propagating along the wire reaches the area of influence of the magnet, a torsional strain is generated in the wire.
Such linear position detectors using a magnet mounted in a float have been utilized as liquid level detectors to provide an indication of a level of liquid within a tank, such as an underground tank. The position of the magnet, and hence, the liquid level, is determined as a function of the time required for a torsional disturbance to propagate from one end of the wire through the area of influence of the magnet, in the case of mechanical excitation, or from the position of the magnet to a sensing apparatus located at one end of the wire in the case of electrical excitation.
Other types of magnetostrictive position measuring devices utilize a reflective termination at the foot end of the magnetostrictive wire. Such devices measure the difference between the propagation times of a pulse from the magnet position to the foot of the wire and reflected back to the head of the device and a pulse traveling on the wire directly from the magnet to the head.
As shown in U.S. Pat. Nos. 4,839,590; 5,017,867; 5,050,430; and 5,253,521, all of which are assigned to the assignee of the present invention, such magnetostrictive devices include an elongated, small diameter tube, typically made of stainless steel, on which a movable magnet is mounted for providing an indication of a liquid level. An enlarged head and cap are mounted on one end of the tube, typically above the liquid level and enclose electronic components, such as terminal connections and a signal conditioning circuit used to supply or output signals to and from the magnetostrictive wire in the tube. As shown in these prior art patents, the head has a considerably larger diameter than the diameter of the elongated tube extending through the tank.
Fluid tanks, such as underground gasoline or fuel tanks, are typically provided with one or more risers or hollow conduits mounted on a top surface of the tank and extending upward to an upper end which is located at ground level. One riser is usually employed for filling the tank and typically will have a diameter of approximately four inches. A second smaller riser, such as a two inch diameter riser, may also be provided as a vent.
Existing tanks which are buried underground and which were not provided with the expectation of using liquid level detection within the tank, typically will not have a large riser or a riser dedicated to liquid level detection. Thus, where it is desirable to provide liquid level detection to such existing underground tanks, previously devised magnetostrictive positioning measuring devices having an enlarged head cannot fit within the fill nozzle of the existing riser on such tanks or will completely fill the interior of the riser thereby preventing the insertion of a fuel nozzle into the riser. The only recourse in such situations is to dig up the existing tank and add a riser specifically for a liquid level detection apparatus.
Thus, it would be desirable to provide a liquid level or position detection apparatus which can be easily mounted in tanks or vessels having small diameter risers. It would also be desirable to provide a liquid level or position detection apparatus which can be easily mounted in existing underground tanks without the need for digging up the tank. It would also be desirable to provide a liquid level detection apparatus in which all of the components of the apparatus are contained within a single, small diameter tube which is extendable into the tank. It would also be desirable to provide a liquid level or position detection apparatus which can be easily mounted on all configurations of sight glasses mounted externally on liquid tanks.